1. Field of the Inventory
The present invention relates to a suspension apparatus having hydraulic cylinders which are arranged respectively between a vehicle body and each wheel and a flow control valve which changes the suspension characteristics by controlling a hydraulic fluid for an operation chamber of the hydraulic cylinder.
2. Description of the Related Art
As disclosed in Japanese Kokai No. Sho 63-130418, conventionally, an active suspension apparatus is well known for controlling suspension characteristics. In a conventional active suspension, the suspension characteristics are changed in a manner such that supply/drain of hydraulic fluid for hydraulic cylinders, which are respectively arranged between a vehicle body and each wheel, is controlled according to the driving state of a vehicle, and, that the amount of fluid in the hydraulic cylinder is changed. The conventional active suspension apparatus comprises a flow control valve for controlling supply/drain of the hydraulic fluid for an operation chamber in the hydraulic cylinder, vertical acceleration detection means for detecting an acceleration of a vehicle body in the vertical direction, lateral acceleration detection means for detecting an acceleration of the vehicle body in the lateral direction, longitudinal acceleration detection means for detecting an acceleration of the vehicle body in the longitudinal direction, integration means for integrating a detected signal of the vertical acceleration detection means, and control means for obtaining a control variable for the flow control valve based on output signals of each detection means and of the integration means. The conventional active suspension is set such that, in the case where a vehicle is rolling, a rolling displacement is suppressed in a manner such that the control signal corresponding to the control variable which is obtained by the control means is outputted to the flow control valve and the PID control (Proportional, Integral, and Derivative Control) is performed on the supply/drain amount of the hydraulic fluid for the hydraulic cylinder.
Furthermore, the detected signal of the lateral acceleration which is detected in the lateral acceleration detection means is differentiated and the rate of change is obtained. The suspension characteristics are controlled in the period when a vehicle is in the transit from a straight driving state to a turning state in a manner such that the control variable of the flow control valve is corrected according to the rate of change of the lateral acceleration. In this manner, the response of the rolling control, which suppresses the rolling displacement of the vehicle, is improved.
With the above described arrangement, there is the advantage that, when the vehicle begins turning, a turning state is quickly detected and the rolling displacement is effectively suppressed. However, in the case where the vehicle is in a stable turning state, e.g. a J-turn (a turn in the shape of J), the controller outputs a signal which drains the hydraulic oil from the operation chamber of the hydraulic cylinder resulting in a tendency to decrease the driving stability.
As shown in FIG. 1A, in the J turn, the actual lateral acceleration G.sub.R gradually increases at the beginning according to a steering operation. Then, in the period t.sub.A-B when the steering operation becomes almost stable, the actual lateral acceleration G.sub.R is somewhat constant, and later, gradually decreases. In the stable turning period t.sub.A-B, as shown in FIG. 1B, the detected value of the lateral acceleration G.sub.D which is outputted from the lateral acceleration detection means is gradually decreased. In the stable turning period t.sub.A-B, the object is to prevent the control signal, which adjusts a height displacement, from being outputted over a long time in response to the output of the lateral acceleration signal including low frequency elements. To attain this object, in the conventional suspension apparatus, the low frequency components of the detected signal G.sub.D of the lateral acceleration are removed by a high-pass filter, therefore, the controller of the suspension apparatus recognizes that the lateral acceleration G.sub.D is gradually decreasing in the stable turning period t.sub.A-B as shown in FIG. 1B.
The phenomenon that the detected acceleration signal G.sub.D is gradually decreased even when the actual lateral acceleration G.sub.r is stable occurs in the case where the vehicle is stopped on a slope.
Therefore, the rate of change of the lateral acceleration G' (=dG/dt) is obtained by the differentiation of the detected value of the lateral acceleration G.sub.D which is necessary for improving a response of the rolling suppression control. As shown in FIG. 1C, a region .alpha. where the value G' becomes a negative value according to the decline of the detected value G.sub.D of the lateral acceleration G.sub.R in the stable turning period t.sub.A-B. In the region .alpha., the controller of the suspension apparatus controls to drain the hydraulic oil from the operation chamber of the hydraulic cylinder resulting in a decrease in the driving stability. Furthermore, as indicated in FIG. 1C at the point t.sub.c where the turn is completed, a region .beta. where the value of the rate of change G' of the lateral acceleration G.sub.D becomes a positive value appears. The region .beta. is originally a region where the rolling control should be suppressed, however, an unnecessary rolling control is executed because the value of the rate of change G' of the lateral acceleration G.sub.D becomes positive, resulting in vehicle instability.